Problem: On the first 6 quizzes of her physics class, Jessica got an average score of 77. What does she need on the next quiz to have an overall average of 79?
Solution: Let her score on the next quiz be $x$ The sum of all of her scores is then $6 \cdot 77 + x$ The same sum must also be equal to $7 \cdot 79$ Solve: $x = 7 \cdot 79 - 6 \cdot 77 = 91$.